Welding power supply with digital controller

ABSTRACT

A welding power supply including power conversion circuitry adapted to receive a primary source of power, to utilize one or more power semiconductor switches to chop the primary source of power, and to convert the chopped power to a welding output is provided. The provided welding power supply includes a pulse width modulated (PWM) digital controller including gate drive circuitry that generates a PWM output signal that controls the switching of the one or more power semiconductor switches. The PWM output signal includes a duty cycle term corrected for one or more sources of error in the welding system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of U.S. patentapplication Ser. No. 13/899,188, entitled “Welding Power Supply withDigital Control of Duty Cycle,” filed May 21, 2013, which is aContinuation Application of U.S. patent application Ser. No. 12/790,423,entitled “Welding Power Supply with Digital Control of Duty Cycle,”filed May 28, 2010, and issued as U.S. Pat. No. 8,455,794 on Jun. 4,2013, which is a Non-Provisional Patent Application of U.S. ProvisionalPatent Application No. 61/183,731, entitled “Welding Power Supply withDigital Control”, filed Jun. 3, 2009, all of which are hereinincorporated by reference in their entireties for all purposes.

BACKGROUND

The invention relates generally to welding power supplies, and, moreparticularly, to a digital controller for a switched mode welding powersupply.

Many types of welding power supplies capable of providing a weldingpower output from an alternating current (AC) or a direct current (DC)source of power have been developed. One such welding power supply is aswitched-mode power supply, which utilizes power semiconductor switchesto chop DC power from a source and convert the chopped power to avoltage and/or current suitable for welding. Switched-mode powersupplies such as inverter type power supplies and chopper type powersupplies have been developed to meet the needs of various weldingprocesses and applications.

The chopper and inverter type power welding power supplies are typicallycontrolled by similar control methods and/or circuits. One method ofcontrolling such power supplies is with a pulse width modulation (PWM)control. A PWM control provides for regulation and control of the outputcurrent and/or voltage of the welding power supply by varying the dutycycle (i.e., the ON/OFF ratio) of power semiconductor switches locatedin the power supply circuitry. Traditional inverter or chopper weldingpower supplies include a closed loop current control loop, such that thepower supply may be operated as a controlled current source for certainarc welding load conditions. As such, traditional inverter or chopperwelding power supplies include an analog controller, which controls theminimum and maximum current levels output from the power source, therates of change of current between various levels, the generation ofdesired current waveforms, and so forth. Unfortunately, analogcontrollers are often associated with drawbacks, such as the inabilityto adequately handle the dynamic requirements of a switched-mode weldingpower supply. For example, analog controllers often fall short ofresponding quickly enough to quickly occurring events in a welding arc,may happen with time intervals of less than 1 msec. Accordingly, thereexists a need for improved control systems and methods for switched-modewelding power supplies.

BRIEF DESCRIPTION

In an embodiment, a welding power supply includes power conversioncircuitry including one or more power semiconductor switches. The powerconversion circuitry is adapted to receive power from a primary sourceand to switch the one or more power semiconductor switches between an ONconfiguration and an OFF configuration to convert the received power toa welding output. The welding power supply also includes a pulse widthmodulated (PWM) digital controller coupled to the power conversioncircuitry and configured to calculate a duty cycle term for control ofswitching of the one or more semiconductor switches by computing anoutput voltage term.

In another embodiment, a welding power supply includes power conversioncircuitry including one or more power semiconductor switches adapted toreceive power from a primary source and to switch the one or more powersemiconductor switches between an ON configuration and an OFFconfiguration to convert the received power to a welding output. Thewelding power supply also includes a pulse width modulated (PWM) digitalcontroller including gate drive circuitry adapted to generate a PWMoutput signal that controls the switching of the one or more powersemiconductor switches. The PWM output signal includes a duty cycle termthat accounts for one or more variations in a bus voltage.

In another embodiment, a digital pulse width modulated (PWM) controllerfor a switched mode welding power supply is adapted to determine anoutput voltage term of the switched mode welding power supply, calculatea variable bus voltage term of the switched mode welding power supply,calculate a proportional error term that corrects for a differencebetween a commanded current level and an actual output current level ofthe welding power supply, and compute a duty cycle term by combining thedetermined output voltage term, the calculated variable bus voltageterm, and the proportional error term.

In another embodiment, a welding power supply includes power conversioncircuitry including one or more power semiconductor switches adapted toreceive power from a primary source and to switch the one or more powersemiconductor switches between an ON configuration and an OFFconfiguration to convert the received power to a welding output. Thewelding power supply also includes a pulse width modulated (PWM) digitalcontroller adapted to sample a current and/or voltage waveform at atrigger location approximately equal to an average of the current and/orvoltage waveform and to calculate a PWM output signal that controls theswitching of the one or more power semiconductor switches based on thesampled current and/or voltage values.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a diagram of an exemplary chopper circuit configured tofunction as a switched-mode welding power supply in accordance withaspects of the present invention;

FIG. 2 is a diagram of an exemplary digital controller for a weldingpower supply that includes gate drive circuitry configured to driveswitching one or more power semiconductor switches in accordance withaspects of the present invention;

FIG. 3 is a flow chart illustrating an exemplary method that may beemployed by a digital controller to calculate and set an appropriateduty cycle for a welding operation in accordance with aspects of thepresent invention;

FIG. 4 is a graph illustrating an exemplary actual current outputwaveform and an exemplary average current waveform that may be generatedat a first output voltage and a first load condition in accordance withaspects of the present invention;

FIG. 5 is a graph illustrating an exemplary actual current outputwaveform and an exemplary average current waveform that may be generatedat a second output voltage and a second load condition in accordancewith aspects of the present invention;

FIG. 6 is an exemplary timing diagram including an exemplary pulse widthmodulated waveform that may be generated by a digital controller inaccordance with aspects of the present invention;

FIG. 7 is a schematic diagram of an exemplary chopper or inverter typewelding power supply system including electrical components of a powersupply and one or more external components in accordance with aspects ofthe present invention;

FIG. 8 illustrates an exemplary unclamped voltage plot and an exemplaryunclamped current plot over time in accordance with aspects of thepresent invention;

FIG. 9 illustrates an exemplary clamped voltage plot and an exemplaryclamped current plot over time in accordance with aspects of the presentinvention;

FIG. 10 illustrates voltage plots including an exemplary filteredvoltage feedback waveform, an exemplary scaled voltage waveform, and anunfiltered fast voltage waveform in accordance with aspects of thepresent invention;

FIG. 11 illustrates a selected region of the unfiltered fast voltagewaveform of FIG. 10 in more detail in accordance with aspects of thepresent invention; and

FIG. 12 illustrates a selected region of the unfiltered fast voltagewaveform of FIG. 10 in more detail in accordance with aspects of thepresent invention.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary chopper circuit 10 configured tofunction as a switched-mode welding power supply 10. The chopper circuit10 includes an AC line voltage input 12, a transformer 14, a set ofdiodes 16, a capacitor 18, a power semiconductor switch 20, a diode 22,an inductor 24, a current sensor 26, an output voltage 28, and a weldingarc 30. The chopper circuit 10 is controlled by a digital pulse widthmodulated (PWM) chopper controller 32 coupled to a weld controller 34.The digital controller 32 includes gate drive circuitry 36 configured toswitch the power semiconductor switch 20 ON and OFF and interfacecircuitry 38, 40 configured to receive current and voltage feedback fromfeedback connections 42, 44, and 46. The weld controller 34 and/or thedigital controller 32 may be coupled to a variety of inputs and outputs,such as the illustrated user interface 48, fan control 50, and thermalsensor 52.

During operation, the AC line voltage 12 is received by the choppercircuit 10 and is transformed by the transformer 14 to a voltage levelsuitable for a welding output. In the illustrated embodiment, thetransformer 14 is a single phase transformer configured to operate atline frequency. In other embodiments, however, the transformer 14 may bea three phase transformer connected to an input source of three phaseline voltage. Indeed, the chopper circuit 10 may be configured toreceive a single nominal input AC line voltage or multiple nominal ACline voltages. As such, in certain embodiments, multiple AC linevoltages may be accommodated by providing taps on the transformer 14,which may be manually or automatically linked for a particular nominalAC line voltage.

An output of a secondary coil of the transformer 14 is rectified bydiodes 16, thus producing a DC bus voltage 54. The capacitor 18 isconfigured to smooth and filter the DC bus voltage 54. As such, in someembodiments, the capacitor 18 may be an electrolytic capacitor, a filmcapacitor, or any other suitable capacitor. The power semiconductorswitch 20 and the diode 22 are configured to function as a powersemiconductor chopper circuit, chopping the DC bus voltage 54. Forexample, the power semiconductor switch 20 is switched ON and OFF by thegate drive circuit 36 located in the digital controller 32 in theillustrated embodiment. As such, the switching frequency and duty cycle(i.e., the ON/OFF ratio) of the power semiconductor switch 20 arecontrolled by the digital controller 32 to provide a regulated outputvoltage and/or current of the welding power supply as dictated by adesired welding process and/or condition. In some embodiments, theswitching frequency may be between approximately 10 KHz andapproximately 100 KHz. For example, in some embodiments, the switchingfrequency may be approximately 20 KHz.

The processed DC bus voltage chopped by the power semiconductor choppercircuit is applied to the inductor 24, which smoothes and outputs theoutput voltage 28. That is, an output current 56 and the output voltage28 are generated and supplied to the welding arc, welding leads, workclamps, and so forth for use in the welding operation. The currentsensor 26 may be utilized to measure the output current 56 and tocommunicate the acquired measurements to the digital controller 32 viathe connection 42. Similarly, the output voltage may be measured andcommunicated to the interface circuitry 40 located in the digitalcontroller 32.

During operation, the digital controller 32 may be configured to controlother functions, such as monitoring thermal sensors 52, controllingcooling fans 50, and bidirectionally communicating various status andcontrol signals to other circuits and controls, such as the weldcontroller 34. For example, the weld controller 34 is configured tooutput a command signal 58 to the digital controller 32. The commandsignal 58 may be an output current level for the welding power supply, acomplex waveform, or a signal dependant on various inputs, such as thewelding process being performed, user inputs received, voltage andcurrent feedback signals, and so forth. As such, the weld controller 34illustrated in the embodiment of FIG. 1 may allow the user to select andcontrol a welding process via the user interface 48. Through the userinterface 48, the weld controller 34 may provide various signals,indicators, controls, meters, computer interfaces, and so forth, whichallow the user to set up and configure the welding power supply asrequired for a given welding process.

The digital controller 32 may be configured to receive one or moreinputs from the weld controller 34 and to utilize such inputs to guidethe functioning of the chopper circuit 10. For example, in oneembodiment, the digital controller 32 may implement a PWM controlscheme. Through a PWM control scheme, the digital controller 32 mayregulate and control the output current and/or voltage of the weldingpower supply by varying the duty cycle of the power semiconductor switch20. In such systems, the welding power supply may include a closedcurrent control loop, such that the power supply may be operated as acontrolled current source for the desired arc welding load conditions.As such, the digital controller 32 may control the minimum and maximumcurrent levels output from the power source, control the rates of changeof current between various levels, and generate the desired waveforms.

Embodiments of the present invention are illustrated herein in thecontext of chopper circuits. However, it should be noted that any of avariety of types of switched-mode power supplies that utilizesemiconductor switches to chop a DC source of power and to convert thechopped power to a voltage and/or current suitable for welding may beutilized with the digital control methods and systems described herein.For example, embodiments of the present invention may utilize any of avariety of suitable inverter type power supplies, such as forwardcircuit, full-bridge, half-bridge, flyback, and so forth. Such powersupplies may also include pre-regulator circuits configured to provide aregulated DC bus voltage to the inverter circuit. Indeed, any of avariety of suitable types or configurations of power supply circuits maybe utilized in conjunction with the digital controller disclosed herein.

FIG. 2 illustrates an exemplary digital PWM controller 32 for a weldingpower supply. The illustrated digital controller 32 includes gate drivecircuitry 36 configured to drive the switching of the powersemiconductor switch 20 of FIG. 1 via a PWM output signal 60. Thedigital controller 32 may also include a variety of circuitry notillustrated in FIG. 2. For example, the controller 32 may includecircuit elements such as analog to digital converters, digital to analogconverters, timers, microprocessors, signal conditioning and filtrationcircuitry, and so forth, which may be used to implement a control schemefor a switched mode welding power supply.

In the illustrated embodiment, the digital controller 32 is configuredto receive a variety of analog inputs 62 including the reference currentcommand 58, a current feedback signal 64, a voltage feedback signal 66,thermal sensor feedback 52, a bus voltage feedback signal 68, and anyother suitable signals 70 that may be utilized by the controller 32 toimplement the digital PWM control or to provide additional functionswithin the welding power supply. That is, the digital controller 32 maybe utilized to perform many of the functions associated with a weldingpower supply that are not directly related to PWM control. Suchfunctions may include thermal monitoring, controlling a cooling fan,controlling status indicators and relays, and so forth. In otherembodiments, however, such peripheral functions may not be performed bythe digital PWM controller 32 and may instead be performed by anothermicroprocessor or control circuit. Nevertheless, in certain embodiments,it may be advantageous to utilize the digital PWM controller 32 toperform such functions in addition to performing the PWM controlfunction. Additionally, the digital PWM controller 32 may interface withvarious other circuits or system components including the fan control50, the weld controller 34, and any other suitable interface devices 72.

In some embodiments, it may be desirable to operate the switched modewelding power supply as a controlled current source, such that thecurrent waveform may be controlled by the welding controller. That is,the welding controller may control parameters such as current level,rate of change of current, lower limit and upper limit current levels,current waveform shape, and other characteristics of the current tocontrol arc characteristics. It should be noted that traditional controlcircuits may implement a high gain or integrating error amplifier withinputs such as a current command or reference waveform and currentfeedback. In such traditional circuits, the error amplifier may generatean error signal, which is traditionally compared to a ramp type signalby a comparator circuit. The output of the comparator circuit is a PWMsignal, which is used to control the power semiconductor switches,thereby controlling the output of the welding power supply.

In some embodiments of switched mode type welding power supplies, thePWM duty cycle (D) may not directly control the output current, but isinstead guided by the following relationship:V_OUT=D*V_BUS, (0<D<1),  (1)in which V_OUT is the output voltage, D is the duty cycle, and V_BUS isthe DC bus voltage. As such, equation (1) is a typical first orderrelationship for duty cycle, output voltage and bus voltage for achopper circuit. It should be noted that a similar relationship may beemployed for an inverter type welding power supply, but may include aterm accounting for the transformer turns ratio, among other factors.

It should be noted that an output current term is not in equation (1),but may be indirectly controlled by the relationship between arc voltageand current or the impedance at the arc in some embodiments. In someembodiments, the arc impedance may vary from a short circuit with lowimpedance to an open circuit with high impedance. Additionally, during awelding operation, the arc impedance may rapidly change (e.g., on theorder of less than 1 mSec). As such, embodiments of the digitalcontroller may vary the duty cycle term in equation (1) depending on thearc impedance. Traditional welding systems including a current erroramplifier may require the control system to detect an error ordifference between the commanded current and actual current before achange in PWM occurs. Embodiments of the presently disclosed digital PWMcontroller, however, may provide an improved PWM control by calculatingand utilizing a variety of suitable terms to generate the necessary PWMduty cycle.

FIG. 3 is a flow chart 74 illustrating an exemplary method that may beemployed by the digital controller 32 of FIGS. 1 and 2 to calculate andset an appropriate duty cycle for a given welding operation. First, thecontroller may calculate the duty cycle (block 76) by employing a firstorder approximation of the relationship between output voltage and dutycycle as defined above in equation (1). However, in further steps of themethod 74, the digital controller 32 may account for a variety offactors present in the welding environment that the first orderapproximation presented in (1) does not incorporate. For example, sinceV_BUS may vary with variations in the AC line voltage supplying thewelding power supply as well as the output power of the welding powersupply, the digital controller may utilize bus voltage feedback tomeasure and account for changes in the bus voltage. In some embodiments,the digital controller may employ a mathematical model for the busvoltage to account for changes in the bus voltage due to line voltagevariations, output voltage, current or power, temperature, or otherfactors.

Furthermore, the digital controller 32 may employ the method 74 toaccount for losses or natural droop in an output volt-amp load linecharacteristic of the welding power supply. Additionally, by employingthe digital controller 32, delays that occur in the power semiconductorswitching circuit, such as gate drive turn ON or turn OFF delays, may betaken into account and utilized to improve system performance ascompared to systems controlled by an analog controller. Specifically,the method 74 includes a step in which the calculated duty cycle may becorrected for the gate drive delay by adding or subtracting a fixed orvariable delay term (block 78). As such, a more accurate model can bederived for the relationship between V_OUT and the duty cycle:V_OUT(t)=(D−D_delay)*V_BUS(t)−(I_out(t)*R_droop),  (2)in which D_delay is the gate drive delay, I_out is the output current,and R_droop represents the losses or natural droop in the outputvolt-amp load line characteristic of the welding power supply. Equation(2) may be rearranged to give an expression for duty cycle:D={V_OUT(t)+(I_out(t)*Rdroop)}/V_BUS(t)+D_delay, (0<D<1),  (3)

The controller may then incorporate feedback signals into the duty cycleequation (3) and rescale accordingly (block 80). Similarly, thecontroller may substitute a commanded output current level for I_OUT(t)in equation (3) because the commanded current level is the targetcurrent level (block 82). As such, a decoupling term (D_dc) may bederived:D_dc={V_fb*K1+I_ref*K2}/{Vbus_fb*K3}+D_delay,  (4)in which V_fb is the feedback voltage, K1 is an appropriate constant,I_ref is the commanded output current level, K2 is an appropriateconstant, Vbus_fb is the voltage bus feedback level, and K3 is anappropriate constant. It should be noted that Vbus_fb may be measureddirectly with a feedback circuit or may be estimated or calculated fromanother signal, such as from an auxiliary power supply winding andcircuit connected to the transformer 14 in FIG. 1 (block 84). As such,equation (4) may by utilized by the controller to set the duty cycleaccording to a current operating condition for output voltage and busvoltage.

Still further, additional corrections may be further made to the dutycycle term to allow for dynamic changes in the duty cycle duringoperation to achieve a desired operating current level or loadcondition. Specifically, an additional term may be incorporated into theduty cycle calculation, which is proportional to the difference betweenthe commanded current level and the actual output current level (block86):D_error=(I_ref−I_fb)*K4,  (5)in which D_error represents the duty cycle correction based on thecurrent error, I_fb represents the feedback current level, and K4 is anappropriate constant. D_error may be positive or negative, and whenadded to the de-coupling term (D_dc) may provide a dynamic way for thedigital controller to adjust the power supply duty cycle to provide acontrolled and regulated current output.

Still further, an integral term (D_integral) may be included to furtherreduce or eliminate steady state error between the actual output currentlevel and the commanded current level (block 88):D_integral=D_integral_previous+K5*D_error,  (6)in which D_integral_previous represents a previous integral term and K5is an appropriate constant. It should be noted that the digital PWMcontroller 32 may be configured to selectively implement the integralterm. That is, the controller 32 may only implement D_integral undercertain conditions, such as when certain welding processes (e.g., GTAW)in which it is desirable to have zero steady state current error areselected by a user. For further example, the controller 32 may implementD_integral term when D_error (the proportional error term) is within abounded range or when the current or voltage output is within a boundedrange. Still further, in some embodiments, the controller may beconfigured to reset D_integral during certain conditions, such as when awelding operation has terminated, when the proportional error term isoutside of a bounded range, or any other preset condition desired by anoperator. Finally, in some embodiments, the integral term may not beimplemented by the controller 32 at all.

The method 74 also includes addition of the calculated duty cyclecorrection terms together to calculate a required duty cycle (D_total)for any given output operating current and load condition (block 90):D_total=D_dc+D_error+D_integral.  (7)In some embodiments, the digital controller 32 may impose furtherupdates on the calculated D_total or its associated terms as desired(block 92). For example, the controller 32 may limit the minimum ormaximum value of D_total. For further example, the controller mayfurther modify the duty cycle terms. In one embodiment utilizing aninverter type power supply, the effect of leakage inductance in the highfrequency transformer may cause an effective delay dependant on thereflected output current. Such a delay may be incorporated into D_totalby providing a variable D_delay term that is dependent on outputcurrent, commanded current, primary transformer current or any othersuitable input capable of accounting for the variable effect of leakageinductance.

FIG. 4 is a graph 94 illustrating an exemplary actual current outputwaveform 96 and an exemplary average current waveform 98 that may begenerated at a first output voltage and a first load condition. Theactual current waveform 96 includes an active “ON” portion 100 and aninactive “OFF” portion 102. The active “ON” portion 100 represents thetime during which power semiconductor 20 is ON, and the inactive “OFF”portion 102 represents the time when power semiconductor 20 is OFF anddiode 22 is conducting. As illustrated, at a point 104 equal to one halfthe “OFF” portion 102, the value of the actual current output waveform96 is approximately equal to the value of the average current waveform98. Furthermore, although the average current waveform 98 represents thedesired current output, the actual output current waveform 96 has a peakto peak ripple 106. An amplitude of the peak to peak ripple 106 may be afunction of a variety of factors, such as features of the power supplysmoothing inductor, the inductance of the welding leads, the outputvoltage, the switching frequency, and so forth.

FIG. 5 is a graph 108 illustrating an exemplary actual current outputwaveform 110 and an exemplary average current waveform 112 that may begenerated at a second output voltage and a second load condition. Asbefore, the actual current output waveform includes an “ON” portion 114,an “OFF” portion 116, a point equal to one half the “OFF” portion 118,and a peak to peak ripple 120. However, at the second output condition,the “ON” portion 114 is shorter than the “ON” portion 100 at the firstoutput condition in FIG. 4. Nevertheless, at the midpoint 118 of the“OFF” portion, the actual current waveform 100 is approximately equal tothe value of the average current waveform 112. FIGS. 4 and 5 furtherillustrate that at approximately the mid point of the “ON” portion ofthe actual current waveforms, 96 and 110, the actual current isapproximately equal to the value of the average current waveform. It isnow realized that such a feature may allow embodiments of the presentlydisclosed digital controller to obtain a single current feedback samplevalue that is synchronized to occur at the midpoint of the “OFF” portionof every switching cycle. It should be noted, however, that inadditional embodiments, a single current feedback sample may be obtainedthat is synchronized to occur at the midpoint of the “ON” portion.

The foregoing feature of the presently disclosed digital controller mayoffer distinct advantages over traditional power supply controllers. Forexample, analog control systems traditionally operate by filtering thecurrent feedback signal to reduce the amplitude of the peak to peakvalue at the expense of phase shifting or adding time lag to the signal.Similarly, digital control theory, which typically guides operation ofdigital controllers, leads to oversampling of the current waveform. Forexample, such a theory may dictate the acquisition of ten or moresamples per period of the waveform and the subsequent calculation of anaverage value based on the ten or more samples. However, with a 20 KHzswitching frequency for the welding power supply, for example, such atheory dictates acquisition of 200,000 or more samples per second, whichwould have to be digitized and averaged to arrive at an accurate averagevalue. Embodiments of the presently disclosed digital controller,however, may acquire a single sample for current feedback that issynchronized to occur every switching cycle at the midpoint of the “OFF”portion of the actual current waveform. As shown in FIGS. 4 and 5, thisis possible because the output current value at the midpoint of the“OFF” portion is approximately equal to the average value of the currentwaveform.

Still further, in additional embodiments, a single sample for currentfeedback may be synchronized to occur every switching cycle at themidpoint of the “ON” portion of the actual current waveform. In otherembodiments, two samples may be obtained, with the first samplesynchronized to the approximate midpoint of the “ON” portion and thesecond synchronized to the approximate midpoint of the “OFF” portion.Further embodiments may average the two samples or selectively obtainand/or use either of the two samples dependant on various operatingconditions such as duty cycle, output current or voltage, or otherconditions.

FIG. 6 is an exemplary timing diagram 122 including an exemplary pulsewidth modulated waveform 124. Such a diagram illustrates how embodimentsof the digital PWM controller disclosed herein may synchronize digitalsampling and conversion to occur at the midpoint of the “OFF” portion byrecalculating a trigger location for an analog to digital conversionbased on the calculated duty cycle value. The value for the triggerlocation for the analog to digital conversion may be further correctedif desired to account for any small phase shift or delay in the currentfeedback signal representing output current or for any time lag that theconversion itself necessitates. That is, the location of the conversionmay be adjusted such that the digitized value will be approximatelyequal to the average value of the waveform. As such, the triggerlocation for the analog to digital conversion (ATD_TRIGGER) may be givenby:ATD_TRIGGER=(1−D)/2+Correction factor,  (8)in which D is the calculated duty cycle and Correction factor is theadjustment factor.

For example, in the embodiment illustrated in FIG. 6, the waveform 124includes a first duty cycle 126 and a second duty cycle 128. The ATDtrigger location is updated during the active duty cycle 126 (e.g., the“ON” portion) by the digital controller (block 130). The digitalcontroller is configured to perform the ATD conversion (block 132) atapproximately one half the “OFF” portion (block 134). After performingthe ATD conversion (block 132) the digital controller is furtherconfigured to calculate a new duty cycle and a new trigger location forthe next period (e.g., duty cycle 128). As such, by sampling andconverting the feedback signals at approximately the midpoint of the“OFF” portion, the duty cycle may be calculated using digitized valuesthat represent the average value of the waveform. Such a method employedby the digital controller also facilitates calculation of the next dutycycle 128 with feedback values that represent a state of the weldingpower supply as well as the desired reference current command, which maybe determined by the weld controller, prior to the onset of the nextduty cycle “ON” portion 128. Such a feature may allow the welding powersupply to be responsive to changes or desired changes during the weldingoperation while reducing or eliminating the time delay as compared totraditional systems.

It should be noted that in some embodiments, it may be desirable toupdate the ATD trigger (sampling) location only once per cycle so thatconsistent inputs are generated and used to calculate the operating dutycycle. Further, it may be advantageous to update the ATD triggerlocation during the “ON” portion as described in detail above such thatthe new trigger location may be implemented by the digital controllerprior to the start of the subsequent “OFF” portion. It should also benoted that in some embodiments, the digital controller may be furtherconfigured to read and convert additional analog signals in asynchronized manner to the PWM waveform. Such additional signals mayinclude voltage feedback, bus voltage feedback, reference currentcommanded by the weld controller, and so forth. The additional signalsmay be used by the digital controller along with current feedback tocalculate the operating duty cycle.

FIG. 7 is a schematic diagram 138 of an exemplary chopper or invertertype welding power supply system. The diagram 138 includes a powersource 140 including an output voltage source 142 controlled by a dutycycle, which is determined by the digital controller, and an inductor144. The inductor 144 is the internal power supply smoothing inductor,which is configured to smooth the output current. An inductor 146 and aresistor 148 represent the equivalent electrical characteristics of theexternal welding leads, which may include the work lead (e.g., theground lead) as well as leads to a wire feeder or other components. Thediagram 138 also includes an arc voltage 150 present between a workpiece and a welding torch or electrode. The diagram 138 also includes anoutput current 152 from the welding power source, which is the currentflowing in the welding arc.

The diagram 138 further includes an arc impedance 154 that representsthe arc impedance at a particular arc condition according to the arcvoltage and output current. It should be noted that the range of the arcimpedance 154 may vary during operation and is dependent on theparticular arc conditions present in the welding operation. For example,prior to arc initiation when current is not yet flowing, the arcimpedance 154 may be large because the output of the power supply is atan open circuit condition. However, during arc initiation, when theelectrode is contacting the work piece, the arc impedance 154 may be lowor even approximately zero. Further, during a welding condition, the arcimpedance 154 may vary due to factors such as the type of weld process,the welding current, operator technique, shielding gas, and so forth. Assuch, the welding arc is dynamic and may vary between a short circuitcondition and an open circuit condition.

It should be noted that the voltage drop across the equivalentinductance and resistance of the weld cable will add to or subtract fromthe arc voltage 150 as observed at the output terminals of the powersource. Such a feature may typically interfere with the ability of theweld controller to control the output voltage 150 and/or the outputcurrent 152. Such interference may also inhibit the ability of the weldcontroller to properly detect the arc voltage, as necessary for certainwelding processes, such as detecting the onset or clearing of a shortcircuit. Such interference may also impact the ability of the digitalPWM controller to accurately calculate a duty cycle term dependant onoutput voltage because the voltage represented in the voltage feedbacksignal, which is derived from the output terminals, may not representthe true arc voltage. However, it is now recognized that a voltage dropacross the equivalent cable resistance 156 is an offset proportional tooutput current 152 and does not change as the configuration of the weldcables changes. The voltage drop across the equivalent cable resistance156 is a function of the cable length, the cross sectional area of thecable, and so forth.

It should be noted that an illustrated voltage drop across theequivalent cable inductance 158 is a function of the time rate of changeof the output current (e.g., first derivative of current):V _(—) L_cable≈L_cable*ΔI_out/Δt,  (9)in which V_L_cable is the voltage drop across the equivalent cableinductance, L_cable is the inductance of the cable, and ΔI_out/Δt is therate of change of the output current. The effects of the voltage dropacross the inductance of the weld cable may vary with the cablearrangement and value of inductance. High levels of induced voltage onthe weld cables may cause ringing (i.e., instability) in the outputcurrent of the welding power supply. As such, embodiments of thepresently disclosed digital controller may be configured to dampen suchringing by clamping the maximum value of the voltage feedback receivedand used for calculating the duty cycle, thereby limiting the effect ofthe induced voltage. That is, embodiments of the present invention mayclamp the value of the feedback voltage used for calculating duty cycleto a suitable value. For example, certain embodiments, the feedbackvoltage value may be clamped to a value that is a preset percentageabove the target voltage or to a preset level that is greater than atypical arc voltage level that may be reached during the given weldingprocess.

FIG. 8 includes an exemplary unclamped voltage plot 160 and an exemplaryunclamped current plot 162. As shown, the unclamped voltage plot 160includes a voltage feedback signal waveform 164 representative of theoutput terminal voltage and an actual arc voltage waveform 166representative of an exemplary scaled true arc voltage. As illustrated,the current plot 162 includes a current waveform 168 that includesringing or oscillation 170 on the peak value. It should be noted thatthe induced voltage across the cable inductance due to the rising edgeof output current may cause the voltage feedback waveform 164 to besubstantially greater than the actual voltage waveform 166 at the arc.In some embodiments, such an effect may cause the ringing 170 evidencedin the output current waveform 168.

FIG. 9 includes an exemplary clamped voltage plot 172 and an exemplaryclamped current plot 174. As shown, the clamped voltage plot 172includes a voltage feedback signal waveform 176 representative of theoutput terminal voltage and an actual arc voltage waveform 178representative of an exemplary scaled true arc voltage. As illustrated,the current plot 174 includes a current waveform 180, which illustratesthe effect of the digital controller directing the clamping of an upperlimit value of the feedback voltage that is used by the digitalcontroller to calculate the operating duty cycle as in equation (4). Itshould be noted that while the actual value of the feedback voltage isapproximately equal in FIGS. 8 and 9, the digital controller isconfigured to clamp the value used in equation (4). As such, the currentwaveform 180 in FIG. 9 does not exhibit the ringing or oscillation 170that is exhibited on the peak of the current waveform 168 in FIG. 8.Accordingly, embodiments of the presently disclosed digital controllerprovide for clamping of the feedback voltage.

Embodiments of the presently disclosed digital controller may be furtherconfigured to reduce or eliminate the effects of the induced voltage inthe voltage feedback signal due to weld cable inductance. As such,further improvements to the digital PWM control and/or the voltagefeedback signal may be obtained with embodiments of the digitalcontroller. Specifically, embodiments of the present invention include amethod that may be employed by the digital controller for measuring orestimating the weld cable inductance during a welding operation andusing this value of inductance to compensate or correct the voltagefeedback signal.

Referring again to the equivalent circuit 138 of FIG. 7, it can be seenthat there are two inductors 144 and 146 connected in series. The inputwaveform to inductor 144 from voltage source 142 is the chopped busvoltage 54 with a duty cycle that is set by the digital PWM controller32. As such, the inductors 144 and 146 form an AC voltage dividercircuit, which splits the high frequency AC component of the chopped DCbus voltage 54 according to their relative inductance values, yieldingthe following equation:Inductance_(—)144/Inductance_(—)146=V _(—)144/V _(—)146,  (10)in which Inductance_144 is the inductance of inductor 144,Inductance_146 is the inductance of inductor 146, V_144 is the voltageof inductor 144, and V_146 is the voltage of inductor 146.

A measurement of the peak-to-peak high frequency (e.g., the switchingfrequency) AC voltage present on the output terminals may be used byembodiments of the digital controller to calculate or estimate theequivalent cable inductance, Inductance_146, if the value of theinductor 144 is known and if the value of the bus voltage 54 is alsoknown or estimated. It is now recognized that the equivalent impedanceof the inductors 144 and 146 at the switching frequency may be greaterthan the impedance of the cable resistance 148 and the arc impedance 154during some or all of a welding operation. As such, embodiments of thepresent invention may provide for the cable impedance or inductance tobe measured or calculated by comparing the relative peak to peak ACvoltage present across each inductor according to equation (9). That is,the input to the inductor 144 is approximately equal to the chopped DCbus voltage. The peak to peak voltage may be measured directly orestimated based on the measured, calculated or estimated DC bus voltage.A measurement of the peak to peak AC voltage signal at the outputterminals of the welding power source may be acquired. Such ameasurement may be utilized by the digital controller to calculate whichportion of the chopped DC bus voltage is dropped across the internalinductor 144 and how much peak to peak AC voltage is dropped across theequivalent weld lead inductance 146. Such a peak to peak AC voltagerelationship may be utilized by the controller along with a known valueof the inductance of inductor 144 to calculate the inductance ofinductor 146. As such, embodiments of the presently disclosed digitalPWM controller may enable the high frequency AC component (i.e., thepeak to peak value) of the output voltage to be measured during thewelding operation and may further enable the measured value to beutilized to determine the equivalent cable inductance.

FIG. 10 illustrates voltage plots that may be generated during a pulseof current, such as the pulse of current shown in FIG. 9. The voltageplots include an exemplary filtered voltage feedback waveform 182, anexemplary scaled voltage waveform 184 representative of the actual arcvoltage, and a relatively unfiltered fast voltage feedback waveform 186.That is, the filtered voltage feedback waveform 182 is representative ofan exemplary signal with an overshoot and an undershoot caused by theinduced voltage of the weld cable inductance. The relatively unfilteredfast voltage waveform 186 represents an unfiltered signal that may begenerated during a first amplifier stage of a circuit that may beutilized to measure the output terminal voltage and provide a scaledfeedback signal for use by the PWM digital controller.

As illustrated in FIG. 10, the unfiltered voltage waveform 186 includesa first region 188, a second region 190, a third region 192, and afourth region 194. The second region 190 and the fourth region 194represent time intervals where there is an approximately stable peak topeak high frequency AC component that may be read by the configureddigital PWM controller and used to calculate or estimate cableinductance. It should be noted that such regions 190 and 194 may also berepresentative of regions where the average voltage and current areapproximately constant, such that the effect of induced voltage due tochanging current may be at a lower limit. It should also be noted thatsuch regions 190 and 194 may also be where the PWM duty cycle is at anapproximate midrange value, which is not near an upper limit value or alower limit value. As such, the second region 190 is shown in moredetail in FIG. 11, and the fourth region is shown in more detail in FIG.12. As illustrated, a peak to peak voltage value 196 of the secondregion 190 and a peak to peak voltage value 198 of the fourth region 194are approximately equal even though the high and low voltage values aredifferent. That is, the second region 190 is offset to a higher overalllevel because it is a region of higher output current and thereforehigher average arc voltage. However, the peak to peak voltages values196 and 198 are still approximately equal.

The first region 188 and the third region 192, on the other hand,represent regions where there is not a valid peak to peak high frequencyAC component that may be utilized by the digital PWM controller. Thatis, the first region 188 and the third region 192 may occur duringdynamic changes in the output operating point of the system, such aswhen the PWM duty cycle is operating near upper limit or lower limitvalues or when the duty cycle has gone to zero and switching of thepower semiconductor in the chopper circuit is not occurring.

Embodiments of the presently disclosed digital controller may beconfigured to measure the peak to peak high frequency AC component shownin the second region 190 and the fourth region 194 to calculate aninductance value for the output weld cable circuit based on the measuredpeak to peak component. As described in detail above with respect toFIG. 6, the digital controller is configured to sample and perform adigital conversion at the midpoint of the “OFF” portion for the currentfeedback signal and the voltage feedback signal, among other signals.Embodiments of the digital PWM controller may be further configured toperform an additional sample and conversion of the unfiltered voltagewaveform 186 at approximately the midpoint of the “OFF” portion and anadditional sample of the waveform 186 at approximately the midpoint ofthe “ON” portion of the waveform.

Such samples of the unfiltered voltage waveform 186 may be acquired andutilized by the digital controller for calculating cable inductance whenthe duty cycle falls within a preset range and/or when the outputcurrent or voltage falls within a preset range. In some embodiments, thedigital controller may restrict sampling of the unfiltered voltagewaveform to regions of approximately constant average current orvoltage. As such, the digital controller may be configured to sample andcalculate cable inductance only during the second region 190 and thefourth region 194 and not during the first region 188 and the thirdregion 192 as shown in FIG. 10. Furthermore, the digital controller maybe configured to acquire such measurements in appropriate pairs. Thatis, if the digital PWM controller detects that the duty cycle fallswithin the preset range to measure the peak value of the AC componentduring the “ON” portion, then a corresponding value will be acquiredduring the subsequent “OFF” portion (e.g., at approximately the midpointof the “OFF” portion). Such acquired values may then be used by thedigital controller to calculate a difference or peak to peak valueaccording to the equation:V_pk-pk={Unfiltered_(—) V(sample 1:Peak value)−Unfiltered_(—)V(sample2:valley or minimum value)},  (11)in which V_pk-pk is the calculated peak to peak value, which is equal tothe first sampled value minus the second sampled value.

The digital controller may be further configured to obtain additionalsamples and calculations for the peak to peak value to obtain a runningaverage or smoothed value over a suitable period of time to reduce oreliminate the potential effects of noise, erroneous samples, and soforth. Subsequently, the digital controller may calculate or approximatethe equivalent cable inductance (i.e., inductance of inductor 146,L_cable) by utilizing the measured value for the voltage bus feedback orany other suitable equivalent signal capable of providing information asto the magnitude of the chopped DC voltage at the input to the inductorand the known value of inductance for the internal power supplysmoothing inductor 144:L_cable=L _(—)144*{(V_pk-pk)/(Vbus_feedback−V_pk-pk)}.  (12)

It should be noted that alternative methods of measuring the peak topeak AC voltage at the output terminals of the power supply andutilizing such a measurement to calculate or estimate weld cableinductance may be employed. For example, the peak to peak AC voltage maybe combined with a measured or estimated bus voltage or peak to peak ACvoltage to the input of inductor 144 to calculate or estimate weld cableinductance. In one embodiment, an analog envelope or peak to peakdetection circuit may be utilized to provide a direct analog valuerepresentative of the peak to peak voltage. Such a value may be utilizedby the digital PWM controller or by the weld controller or othersuitable circuitry to calculate or estimate weld cable inductance 146.Such methods may include steps to selectively utilize the analog peak topeak value during certain periods, such as when the average outputcurrent or voltage falls within a range and/or is at an approximatelyfixed value.

It should be noted that in some embodiments, the peak to peak voltage aswell as the estimated or calculated weld cable inductance value may beutilized by the weld controller exclusively, by the digital PWMcontroller exclusively, by both the weld controller and the digital PWMcontroller, or by other suitable circuitry. That is, in someembodiments, the PWM controller may provide sufficient duty cyclecontrol by clamping the voltage feedback signal. In other embodiments,however, it may be desirable to correct the feedback voltage signal thatmay be utilized by the digital PWM controller by utilizing thecalculated inductance value. In further embodiments, the estimated orcalculated weld cable inductance may be further refined or adjusted bytaking into account the resistance of the weld cable circuit and/or theimpedance of the welding arc.

The digital controller may be configured to utilize the calculatedequivalent cable inductance (L_cable) to correct or compensate thefeedback voltage during the welding operation. That is, referring toequation (9), V_L_cable may then be calculated by the digital controllerby multiplying L_cable by the time rate of change of output current. Forexample, the digital controller may utilize discrete output currentvalues, as measured once per switching cycle, combined with the periodof the switching waveform to calculate V_L_cable.

In further embodiments, the digital controller may use any othersuitable method for calculating V_L_cable. For example, in anotherembodiment, the controller may use a look-up table, which provides anestimation of cable inductance based on the measured peak to peakvoltage feedback value. Additionally, in some embodiments, once a valuefor cable inductance has been calculated by the digital controller, sucha value may be retained after a particular weld sequence has ended andthe arc has been extinguished. The retained value may then be used as aninitial value for the next weld sequence, thereby providing an initialstart value for cable inductance. Subsequent values for cable inductancemay then be recalculated by the digital controller as new data becomesavailable during the welding process.

In further embodiments, a look-up table may be used to provide acorrection factor for the feedback voltage rather than providing aninductance value. Still further, the feedback voltage value(Vfb_corrected) may be calculated according to the following equation:Vfb_corrected=Vfb+K*L_cable*(Ifb−Ifb_previous)/Δt.  (13)

As such, the correction to the feedback voltage may be obtained viaaddition of the first derivative of the current feedback with anappropriate gain value based on measured cable inductance. Theinductance value or voltage feedback correction value determined by anyof the disclosed methods may be communicated to other circuits andcontrols via an analog or digital signal representative of the value ora range of values. In some embodiments, the weld controller may use themeasured or calculated value of inductance in various ways to improvethe welding process control, such as for improving the detection of ashort circuit by using a corrected voltage feedback signal, modificationof various weld process control parameters or waveforms to compensate orcorrect for the weld cable inductance, alerting an operator via a userinterface of a condition when the inductance is outside of an acceptablerange, or any other suitable way. Still further, in other embodiments,the digital controller may be further configured to correct orcompensate for the weld cable resistance. As previously described withrespect to FIG. 7, the weld cable resistance will be exhibited as a DCor offset voltage drop 156 proportional to output current 152. It shouldbe noted that since the resistance value remains relatively constantwith changes in orientation or coiling of the weld cable, a resistancevalue may be measured, calculated, estimated or input to the weldsystem. The resistance value may then be utilized by the digitalcontroller with the value of the output current at any given time tocorrect for cable resistance. For example, relevant data, such as totalweld cable loop length and cable size, may be entered by an operator viaa user interface and used to estimate or calculate the cable resistance.

It should be noted that the digital PWM controller may be furtherconfigured to provide a variety of additional features and benefits overtraditional analog controllers. For example, some welding processes,such as GTAW, utilize low output currents as compared to other weldingprocesses. Traditionally, the output current of the welding power supplyincludes an average value corresponding to a target output currentcommand level and a peak to peak ripple value depending on this averagevalue, as discussed in detail above. For certain processes in which theaverage output current is low, there exists the possibility that a lowerlimit value of the current ripple becomes zero, and the output currentis discontinuous. When such a possibility is realized in a weldingoperation, an arc outage may occur. The digital PWM controller mayreduce or eliminate this possibility by allowing the switching frequencyto be modified based on the selected welding process and/or outputcurrent level. For example, in a low current GTAW process, the switchingfrequency may be increased by a factor of two or three times that of ahigher current process to reduce the peak to peak ripple component,thereby allowing for lower average output current without achieving adiscontinuous output current condition. Furthermore, the digital PWMcontroller may be configured to allow the switching frequency of thepower semiconductor switch to be selectively reduced for certain weldingprocesses or when operating at certain output current levels. Forexample, at high output current levels or in welding processes such asFCAW or CAC, it may be desirable for the digital controller to reducethe switching frequency of the semiconductor switches in order to reduceswitching losses.

Still further, the digital PWM controller may be configured to set lowerlimit and/or upper limit duty cycle ranges, which may be dependent on anoperating condition, such as the weld process, the output current level,or the voltage level. For example, during a dynamic weld condition, itmay be desirable to control the duty cycle to a preset upper limit value(e.g., D_max=0.9) to provide for fast dynamic response of an outputcurrent or voltage. However, it may be desirable to limit the duty cycleto a lower value (e.g., D_max=0.5) for steady state operation, such thatthe maximum continuous output of the welding power supply is limited toreduce thermal stress on the power components.

Additionally, the digital controller may be configured to selectivelyskip switching cycles based on a predetermined output current, apredetermined voltage condition, or a selected weld process. Forexample, the digital controller may skip one or more switching cyclesduring low current and voltages when the duty cycle may be low enough toapproach the gate drive propagation delay times, thus rendering itdifficult to accurately control the output. Alternately, the digitalcontroller may implement a control method where the pulse width (e.g.,the “ON” portion) varies according to the demands of the control systemuntil the pulse width reaches a lower limit value. Beyond this lowerlimit value, the digital controller may implement frequency modulation.

Furthermore, embodiments of the digital controller may be configured tomodify the duty cycle gain or coefficient terms (e.g., K1-K5) based onvarious factors, such as weld cable inductance and/or resistance, arcimpedance, the bus voltage, and so forth, which may impact the overallcontrol loop gain or response. For example, the digital controller maymodify the value of K4 or K5 based on the calculated equivalent arcimpedance, a selected welding process, a target or commanded outputcurrent level, and so forth. Indeed, the digital controller may modifyvarious other factors or coefficients of a compensation function orcontrol loop system to improve the control loop for various operatingconditions.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

The invention claimed is:
 1. A welding power supply, comprising: powerconversion circuitry comprising one or more power semiconductorswitches, wherein the power conversion circuitry is configured toreceive power from a primary source and to switch the one or more powersemiconductor switches between an ON configuration and an OFFconfiguration to convert the received power to a welding output; and apulse width modulated (PWM) digital controller coupled to the powerconversion circuitry and configured to receive one or more analogsignals, to generate an error signal indicative of a steady state errorbetween a commanded current level and an actual output current levelbased at least in part on the one or more analog signals, and tocalculate a duty cycle term for control of switching of the one or moresemiconductor switches based at least in part on the generated errorsignal.
 2. The welding power supply of claim 1, wherein the duty cycleterm is calculated by computing an integral term configured to correctfor the steady state error between the commanded current level and theactual output current level.
 3. The welding power supply of claim 2,wherein the PWM digital controller is configured to selectivelyimplement the integral term based at least in part on the error signal.4. The welding power supply of claim 2, wherein the PWM digitalcontroller is configured to selectively implement the integral term forcertain time periods.
 5. The welding power supply of claim 2, whereinthe PWM digital controller is configured to reset the integral termduring certain operating conditions.
 6. The welding power supply ofclaim 1, wherein the PWM controller is configured to limit the dutycycle term to at least one of a preset minimum value and a presetmaximum value.
 7. The welding power supply of claim 1, wherein the powerconversion circuitry comprises an inverter-type power supply thatcomprises at least one of a forward circuit, a full bridge inverter, ahalf bridge inverter, and a flyback circuit.
 8. The welding power supplyof claim 1, wherein the one or more analog signals comprise thecommanded current level.
 9. The welding power supply of claim 1, whereinthe one or more analog signals comprise a current feedback signal. 10.The welding power supply of claim 1, wherein the one or more analogsignals comprise a voltage feedback signal.
 11. The welding power supplyof claim 1, wherein the duty cycle term is corrected for a gate drivedelay associated with the one or more power semiconductor switches. 12.The welding power supply of claim 1, wherein the duty cycle term iscorrected for a current-dependent or power-dependent loss comprising atleast one of a diode voltage drop, a power semiconductor loss, and aleakage inductance.